The present invention relates to a method of controlling the density of images reproduced by an electrophotographic process and, more particularly, to an image density control method which forms at least two test patterns on a photoconductive element which have greatly different latent image potentials, detects values associated with the image densities of the two patterns or of toner images corresponding to the two patterns, and controls an image density in response to the detected values.
In an electrophotographic or electrostatic recording apparatus, a latent image is formed electrostatically on a photoconductive element by a predetermined procedure and the latent image is developed by fine particles of colored toner supplied from a developing unit. Usually, the toner is charged to a polarity opposite to that of the latent image so that it may be electrostatically deposited on the latent image.
A method available for so charging a toner relative to a latent image employs a developer constituted by a toner and a carrier and stirs them together for frictional charging. This type of developer is usually referred to as a two-component developer. While the developing method using the two-component developer is capable of sufficiently charging a toner to a desired polarity, it requires adequate means for maintaining a constant toner concentration in the developer because only the toner is consumed by the development. It is therefore necessary to measure the varying toner concentration in the developer.
For the measurement of a toner concentration, a somewhat indirect method is known as disclosed in Japanese Patent Publication No. 16199/68. This method comprises the steps of forming a reference latent image pattern electrostatically on a photoconductive drum, developing the reference pattern and photoelectrically measuring the density of the developed image. In a direct method heretofore proposed, on the other hand, the weight or permeability of a developer is measured. Other known methods include one which controls a toner density by detecting a surface potential of a toner image on a photoconductive element (Japanese Patent Laid-Open Publication No. 92138/78). Various other methods have also been proposed for general image density control purpose such as one which controls the bias voltage for development in accordance with a difference in reflectivity between a reference density plate and an original document (Japanese Patent Laid-Open Publication No. 103736/78), one which controls the developing characteristics by detecting an image density during a copying cycle which uses a reference original document (Japanese Patent Laid-Open Publication No. 141645/79), and one which controls the amount of charge on a photoconductive element, bias voltage for development and/or illumination intensity by detecting an image density on an original document, latent image potential and toner image density (U.S. Pat. No. 2,956,487).
One of these known image density control methods employs light and dark patterns, such as black and white patterns, which are electrostatically formed on a photoconductive element. A problem has existed in this type of method in that where the black and white latent patterns on the photoconductive element are developed and the resulting toner densities of the two patterns are sensed by a photosensor, the toner tends to smear the surfaces of light emitting and light receiving elements which constitute the sensor in combination. This, coupled with the deterioration of the coactive elements, effects the input/output characteristics of the sensor so that errors are introduced into the result of measurement.
U.S. Pat. No. 4,082,445 discloses a method which measures a toner density while compensating for the variation in the characteristics of a light receiving element, surface of a photosensitive element and the like due, for example, to a change in the power source voltage, toner deposition, temperature variation and deterioration due to passage of time. In this prior art method, a non-image area on a photoconductive element where the toner is absent is photoelectrically detected first. Because the surface of a photoconductive element has a predetermined reflective power (reflectivity), periodic detection of such a non-image area is effective to see a change in the characteristics of the light receiving element. The change is compensated for by increasing the current which flows through the light emitting element, until the output of the light receiving element returns to a normal value. After the light receiving element has regained its normal density/output voltage characteristic, a density of a reference image is measured to control the toner density. This method, however, invites a disproportionate increase in cost due to the need for an additional circuit for increasing the current supply to the light emitting element. Moreover, the life of the light emitting element becomes shorter owing to the increased load acting thereon.
These drawbacks may be overcome by forming at least two test patterns on a photoconductive element, digitizing values associated with image densities of the test patterns by different resolutions, computing a ratio of the values associated with the image densities in terms of the digital data, and controlling various parameters related with the image densities in correspondence with the computed ratio, as disclosed in Japanese patent application No. 56-178891/81. The different resolutions assigned to the discrete patterns allow the values associated with the image densities to accurately represent the values of the discrete patterns, while the computation of a ratio equally weights the values associated with the respective image densities. Thus, the difference in resolution is equivalent to multiplication or division by a predetermined value. Various parameters related with image densities, therefore, accurately reflect the values associated with the image densities of the different patterns. Furthermore, any fluctuation in the values associated with the image densities attributable to a change in the characteristics of the sensor and photoconductive element appear proportionally in the different patterns, so that the control based on the ratio promotes a stable density control against the variation in characteristic.
Such a method as disclosed in Japanese patent application No. 56-178891/81 still involves a problem due to the use of a microprocessor for computing the ratio (division). As well known in the art, division by a microprocessor requires an intricate computing program and slows down the operation.
The present invention contemplates to omit the division to simplify the computing program although controlling the image density on the basis of the density ratio of at least two test patterns as in the prior art method discussed above. The simpler program will make the construction of a control device simpler and the processing faster.
Suppose that the image density parameter is a supplementary supply of toner, and that a white pattern on a photoconductive element has a developed image density (more strictly, its reciprocal) V.sub.SG while a black pattern has a developed image density (more strictly, its reciprocal) V.sub.SP. Then, in a preferred embodiment, the toner supply is needless when the ratio V.sub.SP /V.sub.SG is smaller than 4/1 due to a sufficient contrast, but necessary when otherwise due to an insufficient contrast. Employing such a threshold value (4/1) and, therefore, V.sub.SP =4V.sub.SG from V.sub.SP /V.sub.SG =4/1, the density can be controlled such that the toner should be supplied when V.sub.SP .gtoreq.4V.sub.SG due to a short contrast but not when V.sub.SP &lt;4V.sub.SG due to an adequate contrast. Thus, whether or not the toner supply is needed can be determined without resorting to division. Meanwhile, when analog signals V.sub.SPa and V.sub.SGa indicative of detected densities are respectively digitized at the ranges of 1:4, the digital data of V.sub. SPa represents V.sub.SP and the digital data of V.sub.SGa, 4V.sub.SG. This permits whether or not to supply the toner to be determined merely by comparing the digital data of V.sub.SPa and V.sub.SGa.
In light of this, the present invention selectively supplies a toner by converting analog data associated with black and white patterns into digital data each in a predetermined range which is different from the other. The ranges can be easily determined by dividing an input analog signal to an A/D converter by a resistance type potential divider.